In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station or network node, e.g. a radio base station, RBS, which in some networks may also be referred to as, for example, “NodeB”, “eNB” or “eNodeB”.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, to communicate with user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations/network nodes connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially flat architecture comprising radio base station nodes without reporting to RNCs.
Precoded Multi-Carrier Signal
FIG. 1 shows an example of precoded multi-carrier signalling. In precoded multi-carrier signalling, a multi-carrier modulator is not directly fed with data in the frequency domain but data is first precoded and then applied to the subcarriers of the multi-carrier modulator. The multi-carrier modulator may, for example, be configured for Orthogonal Frequency Division Multiplexing, OFDM, but may also be configured for any other multi-carrier modulation scheme, such as, e.g. Filter Bank Multi-Carrier, FBMC.
The precoding transformation may be performed by any precoder that enables a certain desired property at the output of the multi-carrier modulator. In many cases, the precoding is used to generate a low Peak-to-Average Power Ratio, PAPR, signal at the output of the multi-carrier modulator. For example, when the multi-carrier modulator is an OFDM modulator, a common choice of precoder is the Discrete Fourier Transform, DFT. Here, the precoded multi-carrier scheme may be the well-known DFT-spread OFDM, DFTS-OFDM, signalling scheme as used in the LTE uplink. According to another example, when the multi-carrier modulator is an FBMC modulator, one choice that reduces PAPR at the output of the FBMC modulator is to perform precoding with a filter bank transformation.
In case of DFTS-OFDM, the output signal for block i may be written as Eq. 1, wherein the subscript i has been omitted for simplicity:y=FHNSFMx,  (Eq. 1)where FM and FN are the DFT matrices of size M and N, respectively, wherein M is the number of allocated subcarrier and N is the IDFT size of the OFDM modulator. The N×M matrix S maps the output of the precoding operations to the Mallocated subcarriers and has exactly one “1” and otherwise only “0” in each column.
For a contiguous mapping of the M subcarriers as described in Eq. 2,
                              S          =                      [                                                                                0                                          L                                              0                        1                                                                                                                                                              I                    M                                                                                                                    0                                          L                                              0                        2                                                                                                                  ]                          ,                            (                  Eq          .                                          ⁢          2                )            where the M×M identity matrix IM and
  0      L          0      1      and
  0      L          0      2      all zero matrices of size L01×M and L02×M, respectively. Non-contiguous mappings such as interleaved mapping are possible as well. The data vector to transmit is the M element vector x.
Typically a guard interval is prefixed to the output signal y to enable simple frequency-domain equalization at the receiver. The guard interval may either be a true guard interval, such as, e.g. an L-element long zero vector, or a cyclic prefix, CP, such as, e.g. a copy of the last L elements of y, as is shown in FIGS. 2-3. In both these cases, the signal with guard interval can be written as Eq. 3:{tilde over (y)}=Py=PFHNSFMx,  (Eq. 3)where P is the matrix inserting the true guard interval or cyclic prefix.Frame Structure
Transmissions in wireless communication networks are often organized in terms of frames or subframes. Each frame or subframe is a group of transmission resources, e.g. time-frequency resources, which comprise both at least one control field and at least one data field, i.e. field for payload data. Typically, the control field appears in the beginning of the frame or subframe and comprise control information, such as, e.g. information about how the data in the data field of the frame or subframe is encoded and modulated. The control field may also comprise control information related to data transmission in the reverse link direction, i.e. data transmitted from the receiver of the control information, such as, e.g. ACK/NACK reports or Channel State Information, CSI.
A possible frame structure of a wireless communication network is illustrated in FIG. 4. Any two communication nodes which are communicating may in principle arbitrarily select which of the two control fields should be used for transmitting, Tx, and which of the two control fields should be used receiving, Rx. This is shown in the upper left and right illustrations in FIG. 4.
However, this arbitrary selection may require complicated negotiation procedures between the communication nodes, and hence it is often more practical to have a general rule for the system. One example of this is shown in the lower left and right illustrations in FIG. 4, where one of the control fields is always used for downlink transmission, DL Tx, such as, e.g. transmission by access or network nodes in the wireless communication network, and the other control field is always used for uplink transmission, UL Tx, such as, e.g. transmission by wireless devices.
Note also that frames on different links in the system should preferably be time-aligned, partly because this enables communication nodes, such as, wireless devices and network nodes, to more freely and efficiently change the communication node with which it communicates from one frame to another, e.g. without waiting for the another communication link to finish its frame.
The control and data fields are often further divided into smaller units. For example, in an OFDM-based wireless communication networks, the control and data fields are further divided into one or more OFDM symbols. This is also true for other networks, such as, e.g. networks based on DFTS-OFDM, FBMC, etc. Hereinafter, such units may be referred to as symbols. Some fields may comprise only a single symbol.
Half-Duplex
If a wireless communication network does not use a paired spectrum, i.e. different frequency bands for the two link directions, as millimeter-wave, mmW, networks typically don't do, it is normally necessary to limit communication to half-duplex. Half-duplex means that transmission may at any one time instance occur only in one of the two link directions, i.e. directions of communication, such as, transmitting or receiving.
Hence, in this case, Time-Division Duplex, TDD, has to be used. One reason for this limitation is that a communication node that is transmitting, such as, e.g. an access or network node or wireless device, saturates its own analog receiving circuitry due to strong overhearing between transmit and receive antennas. In half-duplex systems, it may be useful to have two control fields for control information in every frame, one for each link direction. The two directions of a link may be referred to as Tx/Rx directions or duplex directions. In other words, a communication node may use one of the control fields for Tx and the other control field for Rx.
It should also be noted that within each of the three fields, i.e. the two control fields and the data field, there may typically also be other signals interspersed, such as, e.g. reference or pilot signals carrying reference signal information, to allow the receiver to perform channel estimation.
As shown in FIG. 4, each field typically comprises reference signal information plus data and/or control information. In case of OFDM multi-carrier signaling, this information may be multiplexed into the same symbol by using different subcarriers.
However, when having a precoded multi-carrier signal, such as, e.g. DFTS-OFDM, assigning one set of frequencies to reference signal information and another set of frequencies to data and/or control information will increase the Peak-to-Average Power Ratio, PAPR. This is a problem since a low PAPR is the reason for using precoded multi-carrier signaling in the first place.
One solution is to dedicate or reserve at least one OFDM symbol to reference signal information and at least another OFDM symbol to data and/or control information. However, a complete OFDM symbol dedicated or reserved for strictly reference signal information will result in a large signaling overhead for the reference signal information.